NASA Model-Based Systems Analysis and Engineering Final Report

The traditional approach for aircraft mission analysis using the familiar force decomposition into lift, weight, thrust, and drag faces challenges when applied to novel configurations that exhibit strong aero-propulsive coupling. A clear, unambiguous force decomposition is not always possible. The research discussed in this report presented an alternative formulation to the equations of motion that is more generalizable and amenable to modern trajectory optimization strategies. The force decomposition required by the traditional approach is no longer necessary.
A new tool called Olympus was developed that conducted mission analysis using this kinetic formulation of the equations of motion. To demonstrate this new approach and assess the benefits and drawbacks over the traditional methodology, two use cases were defined. The first use case was a conventional tube and wing vehicle that is intended to be a notional representation of an Airbus A320neo single-aisle aircraft. The second use case was a more advanced, novel, single-aisle tube and wing called the Strut Braced Wing. The Strut Braced Wing is intended to be a notional representation of Boeing’s Transonic Truss Braced Wing (now X66) concept, developed as part of the NASA-funded Zero Emissions project. However, unlike the X66, the SBW is powered by a large Open Rotor. This rotor wake influences the airflow over the wing and strut, thereby resulting in stronger aero-propulsion interactions relative to a conventional vehicle.
The results from the benchmark A320neo runs verified the Olympus tool’s ability to solve the vehicle sizing problem by comparing the results generated by the Aviary tool and FLOPs. With the exception of the ground segments and minor differences in how the constraints were setup, the Olympus tool’s results match closely with those from Aviary and FLOPS.
The SBW use case was run with three sets of aerodynamics data. The first set was a table of CD vs. CL, Mach and altitude, where the the lift and drag coefficients were computed on the SBW airframe that experienced the SROR wake. As such, the aero-propulsion interactions were accounted for in the aerodynamics data. This was called the “weakly-coupled” approach. In the “uncoupled” data set, the SROR was removed from the CFD model and the mission drag polar data was re-generated. As such, this set did not account for the aero-propulsion interactions. Finally, the third data set (“strongly-coupled”) modeled the net streamwise and stream-normal force coefficients as a function of Mach, Reynolds number, power code, and angle of attack. These coefficients added the SROR thrust contributions resolved in the x and z directions of the wind axes to the CL and CD calculations. The FLOPS analysis for the SBW was run with both the uncoupled and weakly-coupled data sets, while the Olympus runs used all three data sets.
The strongly-coupled results from Olympus show a 2% higher design mission fuel burn compared to the weakly coupled results, suggesting that the decomposition of the net streamwise force into thrust and drag is not as critical for this aircraft. However, when comparing the strongly-coupled Olympus design mission fuel burn to the non-coupled Olympus mission fuel burn, a 14% difference is observed. In other words, ignoring the aero-propulsion interactions for this configuration when generating the aero data tables results in a 14% under prediction of fuel burn, which is substantial. The difference between the FLOPS results (weakly coupled relative to uncoupled) and the Olympus results for the same pair are comparable at 10% and 12% respectively. In short, the results clearly demonstrate that for concepts with stronger aero- propulsive interactions, the traditional uncoupled approach for handling the aerodynamics and propulsion disciplines separately can result in a substantial mis-estimation of the performance.